In a seat back which is a backrest of a vehicle seat, there has been disposed an armrest which is provided with a recess portion opened forward and can be switched between a state of being accommodated in the recess portion and a state of projecting forward from the recess portion. In a seat back of an automobile seat disclosed in JP-A-2015-47387, a resin board is utilized in order to mount a back cover, which is a surface material, on a recess portion provided in a back pad as a cushion body in a good appearance. Specifically, the resin board is mounted on a back surface of the back cover corresponding to a rear wall portion of the recess portion, and thus the back cover is fixed by being drawn into the recess portion of the back pad via the resin board. As a fixed structure, the back cover is mounted by being sewn to an end portion of the resin board, and the end portion of the resin board is inserted into a fitting groove provided in a side wall portion in a rear wall outer periphery of the recess portion of the back pad. Further, a tongue piece is disposed through a hole provided in the back pad, and one end of the tongue piece is fixed on the resin board and the other end of the tongue piece is fixed on the back surface of the back pad, so as to tightly fix the resin board on the rear wall of the recess portion of the back pad.
In the related art, the end portion of the resin board is inserted into the fitting groove provided in the side wall portion of the recess portion in order to fix the resin board on the rear wall of the recess portion of the back pad. However, engagement strength is not enough only by inserting the end portion of the resin board into the fitting groove provided in the side wall portion of the recess portion. Therefore, fixation with a tongue piece is added for sufficient engagement strength. A reason that the strength cannot be ensured only by inserting the end portion of the resin board into the fitting groove is that, a depth of the fitting groove cannot be molded to be a predetermined depth or more from a necessity of demolding after molding since the fitting groove is an undercut part in the back pad. As a result, there is a problem that a structure is complicated and manufacturing workability of the seat back is poor since the fixation with a tongue piece is also necessary while providing the fitting groove in the back pad.